A cathode ray tube (CRT) has been widely used as one of display devices for displaying image data on a screen. However, the CRT has been inconvenient for use because it has a large volume and a heavy weight for its display area.
A thin flat display device, which has a slim profile and a large display area and thus can be used conveniently anywhere, has been developed and is gradually replacing the CRT.
A liquid crystal display device (LCD) is driven using characteristics of liquid crystals, optical anisotropy and polarization. Since liquid crystal molecules are long and thin, the alignment of liquid crystal molecules has directivity. The alignment direction of the molecules may be controlled by artificially applying electric field to liquid crystals. Therefore, light can be transmitted or blocked according to the direction alignment of the liquid crystal molecules through the optical anisotropy of liquid crystals to display a color and an image.
Since the LCD cannot emit light by itself, it displays an image using light provided from an external light source or a backlight unit.
FIGS. 1 and 2 are an exploded perspective view and a partial enlarged view for illustrating a coupling structure of a light guide plate and a frame of a related art backlight unit.
Referring to FIGS. 1 and 2, the related art backlight unit includes a frame 10 and a light guide plate 20. A first recess 11 and a second recess 13 are formed at an inner side of the frame 10. The first and second recesses 11 and 13 are alternately formed and face different directions from each other. That is, the first recess 11 is formed on an upper surface of the frame 10, while the second recess 13 is formed on an under surface of the frame 10. A first protrusion 21 and a second protrusion 23 are formed at a side surface of the light guide plate 20. The first and second protrusions 21 and 23 are alternately formed and face different directions from each other. That is, the first protrusion 21 is formed at an upper portion of the side surface of the light guide plate 20, while the second protrusion 23 is formed at an under portion of the side surface of the light guide plate 20.
In the backlight unit having the above-mentioned structure, the first recess 11 is coupled to the first protrusion 21 and the second recess 13 is coupled to the second protrusion 23, and thereby coupling the frame 10 to the light guide plate 20.
FIG. 3 is a view for illustrating a design criteria of a related art backlight unit.
Referring to FIG. 3, the related art backlight unit may be divided into a light guide plate region, a light emitting region, and an image display region. The light guide plate region may be defined by a width aW and a length aL, of the light guide plate, the light emitting region may be defined by a width bW and a length bL of the light emitting region, and the image display region may be defined by a width cW and a length cL of the image display region. The image display region corresponds to an active region of the LCD.
The light emitting region is approximately 0.5 mm larger than the image display region. The light guide plate region is approximately 0.5 mm larger than the light emitting region. An opening region in the middle of the frame 10 is approximately 0.05-0.1 mm larger than the light guide plate 20.
In a small-sized backlight unit having the above-mentioned structure, light emitted from a lateral side of the light guide plate 20 may be reflected on a sidewall of the frame 10 and be incident to the light guide plate 20. Here, light of a band shape delicately shows on an outer edge of the light emitting region due to light reflected on the sidewall.
Also, the light guide plate having a thickness of 0.3 mm or smaller has been required, but it is difficult to form protrusions for coupling such a thin light guide plate to a frame. Although the protrusions are formed, they may be broken due to their frangibility. When the protrusions are broken, the broken pieces move into the light emitting region to affect image display.